The present invention relates generally to radio communication devices, and specifically to crystal based devices and methods for accurate frequency control of radio transceivers.
Crystal oscillators (XOs), which are often used in frequency control on radio transceivers and other applications, are not in and of themselves sufficiently stable to provide an accurate and consistent source signal. The inherent frequency of a crystal oscillator is known to vary with temperature and also to change gradually as the crystal ages. Because of these shortcomings of ordinary crystal oscillators, transceivers known in the art generally use either a temperature-compensated crystal oscillator (TCXO) or a voltage-controlled TCXO (VCTCXO) as a precise frequency reference source. TCXO, and VCTCXOs, however, are larger and more expensive than simple crystal oscillators.
Systems and methods are known which derive control parameters, such as divider values, for a conventional synthesizer from the synthesizer""s crystal coefficient values and from its temperature. These systems are able to obtain from a frequency synthesizer using a conventional crystal oscillator frequency accuracy comparable to that of TCXOs or VCTCXOs based synthesizers. More specifically, these systems take into account that the output frequency of an XO equals a nominal output frequency (f0) plus xcex94fCr which may be calculated in accordance with the formula below:
xcex94fCr [ppm]=d+c*tx+b*tx2+a*tx3 
where: tx is equal to the actual temperature (Tactual) minus the nominal crystal constant (Tnominal), the crystal coefficients (d, c, b, and a) are specific to the crystal, and xcex94fCr [ppm] represents frequency deviation in parts per million.